1. Field Of The Invention
The invention relates to a biaxially stretch-oriented polyester film, in particular a biaxially oriented poly(ethylene terephthalate) film (BO-PET film) which, due to its abrasion and adhesion behavior toward coatings, is particularly suitable as a base film for magnetic tapes.
2. Prior Art
As is known, films of this type (BO-PET) are provided with an adhesion-promoting coating, applied in an aqueous dispersion or in an organic solvent.
It is also known, from U.S. Pat. No. 4,524,108, to treat a poly(ethylene terephthalate) film with a corona discharge treatment before application of the magnetic layer. In this reference, however, the corona discharge treatment only results in an improvement in the adhesion if the proportion of molecules in the vertical direction, i.e., the proportion of molecules oriented toward the coating on the film is particularly large. The refractive index serves as a measure of the proportion of vertical molecules. According to this publication, the adhesion is particularly good if the refractive index in the direction of the thickness of the biaxially stretch oriented poly(ethylene terephthalate) film is at least 1.4990.
It is evident from the comparison experiments of this reference that significant delamination of the magnetic layer occurs, in spite of corona treatment, at values of 1.4946 and 1.4905 for the refractive index. A refractive index of at least 1.4990 can only be achieved in "balanced"BO-PET films. "Balanced"means that the mechanical properties are approximately equal both in the longitudinal direction and the transverse direction and are at a relatively low level. The moduli of elasticity measured in the longitudinal and transverse directions are of approximately equal magnitude and are about 5800 N/mm.sup.2 (Newtons per millimeter squared).
In BO-PET films having a relatively high modulus of elasticity in the longitudinal film direction (greater than 6000 N/mm.sup.2), the orientation in the plane is so high that the perpendicular refractive index is significantly below 1.4990, even below 1.4900. According to U.S. Pat. No. 4,524,108, films of this type are said to have poor abrasion resistance and poor adhesive strength to magnetic coatings, even if they have been subjected to a corona discharge. This also applies to the coatings applied on the "reverse"or underside of the film for magnetic tapes, which coatings are intended to ensure good running behavior of base films having particularly smooth surfaces on the "reverse"surface (the "reverse"surface of the film is the surface typically having no magnetic layer). The binder coatings which are customary for the reverse surface (for example, polyurethane resins) are thus even more critical with respect to adhesion to the film surface than are the binder coatings for the magnetic layer.
The object of the present invention is thus to find a biaxially oriented polyester film which is suitable, without an adhesion-promoting layer, as a base film for magnetic coatings, i.e., has good abrasion resistance and adhesion properties, and good mechanical properties. This object is achieved by the film of the present invention.